It is known that manganese dioxide of a grade suitable for use as a depolarizer in dry cell batteries can be manufactured synthetically from precursor manganese sulfate solutions by the oxidation, either electrolytically or by use of an oxidizing agent, of the manganese ion in said solutions to manganese dioxide. Further, it is known that the precursor manganese sulfate solution can be prepared by the reduction and digestion of naturally occurring manganese ores such as those of the pyrolusite/cryptomelane type. However, a drawback to the use of such manganese ores and particularly those ores of the cryptomelane type is the presence therein of high levels of potassium.
The presence of potassium impurity in electrolytic manganese dioxide adversely affects the latter's capability to function satisfactorily as a depolarizer in dry cell batteries. As a result, numerous solutions have been proposed for removing potassium impurity from electrolytic manganese dioxide. The majority of these proposed solutions relate to the removal of potassium impurity during the preparation of the precursor manganese sulfate solution. For example, in U.S. Pat. No. 3,667,906 there is disclosed a method of removing potassium impurity from naturally occurring manganese ores used in the preparation of the manganese sulfate electrolyte comprising washing a reduced or calcined manganese ore with superheated hot water to remove soluble potassium prior to subjecting said reduced or calcined manganese ore to subsequent acid leaching and separation steps.
U.S. Pat. No. 4,285,913 describes a process for producing manganese sulfate electrolyte solutions containing reduced concentrations of potassium. In this patent, there is described a process wherein a reduced or calcined manganese ore containing potassium impurity is leached with a liquor comprising spent electrolyte (i.e., an aqueous solution containing sulfuric acid and manganese ion) to which has been added a source of iron to provide soluble ferric ions. The leaching process is carried out under such specific conditions of pH, time and temperature at to yield a manganese sulfate solution disclosed as having reduced levels of potassium impurity. Subsequent treatment of this solution with additional reduced or calcined ore to raise the solution pH of the mixture and to precipitate the remaining iron thus provides an electrolyte highly suited to the preparation of electrolytic manganese dioxide.
Although not mentioned in U.S. Pat. No. 4,285,913, the invention therein described apparently makes use of the "jarosite process", named after the resulting iron precipitate, as described in U.S. Pat. Nos. 3,434,947 and 3,493,365 and employed in the zinc industry for the recovery of zinc and other valuable metals by electrolytic processes. In this process, small amounts of ammonium, sodium or potassium ion are added to a leach liquor resulting from the leaching of a zinc calcine, neutralized and the iron impurity present in the solution precipitated out, as jarosite having the formula EQU (NH.sub.4, Na or K)Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6.
Another process for producing manganese sulfate solutions having reduced concentrations of potassium as an impurity is the improved process described in copending and commonly assigned U.S. application Ser. No. 577,996 and entitled "IMPROVEMENTS IN THE MANUFACTURE OF MANGANESE SULFATE SOLUTIONS". The improvement described therein is based on the discovery that when a digestion mixture of a reduced manganese ore containing potassium impurity, a source of water soluble ferric ions and an aqueous acid solution of, for example, a spent electrolyte is digested at elevated temperatures in the presence of a preformed potassium-iron salt, an accelerated rate and extent of removal of the potassium impurity can be achieved. The use of the preformed salt also allows for employment of lower temperatures during the digestion of the mixture although the solution pH still must be maintained between about 0.5 to about 3.5 to take advantage of the presence of the preformed salt.
Yet another process for producing manganese sulfate solutions having reduced concentrations of potassium impurity is the process described in copending and commonly assigned U.S. application Ser. No. 577,995 and entitled "METHOD OF PRODUCING MANGANESE SULFATE SOLUTIONS OF IMPROVED PURITY". In the process described therein a digestion mixture of reduced manganese ore, a source of added water soluble aluminum ions and an aqueous acid solution, such as spent electrolyte first is prepared and then digested. The resulting mixed reaction product comprises a liquid phase of manganese sulfate solution and a solid phase of digested ore residue and a potassium-aluminum salt byproduct which is formed by reaction of potassium impurity with the aluminum ions. The advantage to using aluminum ions is that significantly less acidic conditions need be employed. In fact, mixtures having solution pH's of at least 4.0 must be employed to effect removal of potassium impurity during digestion of the mixture. Another feature of the process described in the above application is the use of the potassium-aluminum salt byproduct as a means to reduce the induction period for the reaction between the potassium impurity and aluminum ions in the digestion mixture.
While the processes of the above-noted patents and applications are effective in reducing the concentration of potassium impurity in manganese sulfate solutions there still exists a need for processes which will provide even further improvements in the rate at and extent to which potassium removal can be achieved. The process comprising the present invention provides such further improvements.